Using a random deal of cards to players and a computationally unlimited eavesdropper, all players wish to share a one-bit secret key which is informationtheoretically secure from the eavesdropper. This can be done by a protocol to make several pairs of players share one-bit secret keys so that all these pairs form a tree over players. In this paper we obtain a necessary and sufficient condition on the number of cards for the existence of such a protocol.